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Solutions to Membrane Fouling
By G.K. Dhawan, 1985
Introduction to Membrane Fouling
All membranes lose their performance with time. One of the major causes for the loss of performance is due to substances that deposit on the membrane surface. Although the term fouling is used for deposit of any materials on the membrane, the coating of the membrane surface can be due to the following:
Fouling of membranes is due to the suspended or emulsified materials that may be present in the feed water to the reverse osmosis system. Examples of such materials are: silica, oil, clay, iron, sulfur and humic acids. These substances can be present in a very fine or colloidal form. Even the typical 5 micron cartridge filters used upstream from a reverse osmosis system may not completely remove these foulants.
Membrane Fouling and Scaling
The concentration of all materials in the feed water - dissolved and suspended - is highest near the membrane surface. As permeate is removed through the membrane, all impurities are left behind near the membrane surface. The layer of water next to the membrane surface (boundary layer) gets more and more concentrated in the dissolved and suspended materials. These concentrations reach a certain steady level depending on feed velocity, element recovery and membrane permeate flux (gallons per square foot of permeate produced per day).
It is important to follow membrane manufacturers' recommendations on minimum feed flow, maximum element recovery and maximum element flux. These recommendations are based on the element size and quality of feed water being treated. The concentrations of the dissolved and suspended solids in the boundary layer control the performance of the membrane. Higher concentrations mean higher osmotic pressure, higher tendency of suspended solids to coagulate and coat the membrane surface, and higher likelihood of scaling to take place. Maintaining proper operating conditions for the membrane is the key preventative step to minimize membrane fouling.
For non-residential systems, another option to avoid calcium carbonate and calcium sulfate scaling is by the use of antiscalants. These are injected directly into the feed water upstream from the cartridge filter. Dosage of antiscalant depends on the feed water analysis but usually is between 2 to 5 ppm. In simplified terms, the antiscalants delay the scale formation process. This delay is sufficient to avoid precipitation of calcium carbonate and calcium sulfate on the membrane surface. As this delay is for a finite period, scaling can take place in systems on shut down. For this reason, it is a good practice to flush the membranes with permeate or feed water at shut down. By this flush, the concentrated solution in the membrane is displaced by the permeate or feed water.
For suspended or colloidal materials, a dispersant can be injected in the feed water. The usual dosage for a dispersant is 10 ppm. Dispersants keep fine suspended solids from coagulating and coming down on the membrane surface. Proper use of dispersants can minimize fouling due to problem particulates that are difficult to prefilter.
Adjusting the pH of the feed water is another way to control calcium carbonate scaling. The net effect of lowering the feed pH with acid injection is to convert bicarbonate alkalinity to carbon dioxide and thereby prevent the formation of calcium carbonate. For reasons of handling and safety, acid injection is not used for residential or small commercial systems.
Membrane recovery is defined as the ratio of permeate flow to feed flow for that membrane. Recovery can be reduced by increasing the feed flow. Another way to reduce recovery is to decrease the operating pressure. Lower operating pressure produces a lower amount of permeate. If the feed flow can be maintained near the original value, then a lower recovery is obtained.
The effect of lower recovery is to reduce the overall concentration of all substances in the reverse osmosis system. More favorable boundary layer conditions are also achieved by reducing the system recovery.
Even with all the preventative care given to a reverse osmosis system, some fouling of the membranes will take place. Cleaning of the membranes can improve membrane performance. Membranes can be cleaned using Cleaning Solutions approved by the membrane manufacturer. It is not economical to clean membranes used in the residential reverse osmosis systems.
Membrane fouling and scaling can be minimized by proper design and operating conditions. Important variables that control the membrane fouling must be considered in designing an operating the reverse osmosis system.
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